1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus such as a copying machine, a facsimile apparatus, and a laser printer. More particularly, the present invention relates to an image forming apparatus having a subsidiary charge roller which allows developer attached to a development roller to be evenly charged.
2. Description of the Related Art
In general, an electrophotographic image forming apparatus such as a laser beam printer, copying machine and facsimile apparatus, obtains a permanent image formed by the steps of first contacting developer containing fine toner particles with a development roller such that the developer is attached to the development roller, and then rendering the toner particles contained in the attached developer to be selectively attached to an electrostatic latent image by electrostatic force using suitable voltage bias, thereby forming a toner image. The electrostatic latent image is preferably formed on a photosensitive conductor which is formed from a conductive material. The apparatus then transfers the toner image to a paper which is more highly charged than the photosensitive conductor, and fixes the toner image using heat and pressure.
FIG. 1 shows a cross-sectional view of such a conventional image forming apparatus.
The image forming apparatus 10 comprises a photosensitive conductor 1 in the form of a cylindrical drum rotating in a predetermined direction, as indicated by arrow A in FIG. 1, with a predetermined shaft speed. The apparatus further includes a charging unit 2, such as a contact charge roller, for charging the surface of the photosensitive conductor 1 to a predetermined potential. An exposure unit 3 is provided for scanning laser beams onto the surface of the photosensitive conductor 1 in response to an image signal, thus forming an electrostatic latent image.
A development unit 4 is provided for attaching developer 18 to the electrostatic latent image formed on the surface of the photosensitive conductor 1, thereby developing the developer 18 into a visible image. A transfer unit 9 is then provided for transferring the developer 18 developed into the visible image onto a paper 13, and a fixation unit 12 is provided for fixing the developer 18 transferred onto the paper 13 using heat and pressure. A paper discharge unit 14 is finally provided for discharging the paper out of the apparatus.
The development unit 4 comprises a development roller 5 spaced from the photosensitive conductor 1 by a developing gap ‘d’, and a developer supply roller 6 for supplying developer 18 to the development roller 5, wherein the developer supply roller 6 is located adjacent to the development roller 5 and rotating in a same direction with the development roller 5 as indicated by arrow B in FIG. 1. The unit 4 further includes a stirring roller 16 for stirring developer 18 contained in a developer bin 17, and a developer layer regulation member 7 for regulating a developer layer formed on the development roller 5. A subsidiary charge roller 8 is provided for more evenly charging the developer layer formed by the developer layer regulation member 7, and a leakage prevention member 15 is provided for preventing the developer from leaking out, wherein the leakage prevention member is located below the development roller 5.
The image forming apparatus 10 constructed in this manner can be operated as follows. In a first step, the photosensitive conductor 1 having a surface being charged with a predetermined voltage via the charging unit 2, rotates in the direction indicated by arrow A in FIG. 1.
At this time, laser beams emitted from the exposure unit 3 are scanned onto the photosensitive conductor 1 in response to an image signal, whereby an electrostatic latent image is formed on the surface of the photosensitive conductor 1.
The developer 18 is then moved from the developer bin 17 by the stirring roller 16, to the developer supply roller 6, which is rotating in the direction indicated by arrow B in FIG. 1.
The developer 18 which arrives at the developer supply roller 6, is moved to and attached to the development roller 5 due to a difference in potential between a developer supply bias voltage (for example, −400V to −800V) applied to the developer supply roller 6, and a developing bias voltage (for example, −200V to −600) applied to the development roller 5. The developer 18 is moved into a nip between the development roller 5 and the developer supply roller 6, wherein the development roller 5 rotates in the same direction with the developer supply roller 6.
At this time, although the developer 18 is charged with a negative (−) polarity by the developer supply roller 6 and the development roller 5, the developer 18 contains a mix of toner particles, some charged with a negative (−) polarity and some non-charged (neutral) particles.
Thereafter, as the development roller 5 continuously rotates, the developer 18 is formed into about one to six developer layers on the surface of the development roller 5 by the developer layer regulation member 7. The developer 18 formed on the surface of the development roller 5 then moves into the nip between the subsidiary charge roller 8 and the development roller 5 after, or downstream of the developer layer regulation member 7 in the rotational direction of the development roller 5, wherein the subsidiary charge roller 8 rotates in the direction opposite to that of the development roller 5.
At this time, all the toner particles existing in the developer 18 are charged with a negative (−) polarity because the non-charged toner particles noted above are now charged by the subsidiary charge roller 8, to which a voltage in the range of about −500 V to −2000 V is applied.
The developer 18 then moves to the surface of the photosensitive conductor 1 in the developing area where the developing gap ‘d’ is formed between the development roller 5 and the photosensitive conductor 1. At this point, the developer 18 is attached to an electrostatic latent image formed on the surface of the photosensitive conductor 1, thereby being developed into a visible image.
As the photosensitive conductor 1 rotates, the developer 18 attached to the photosensitive conductor 1 is transferred to a paper 13, which is fed between the photosensitive conductor 1 and the transfer roller 9 located below the photosensitive conductor 1.
As the photosensitive conductor 1 continues to rotate, non-transferred developer left on the latent image area on the surface of the photosensitive conductor 1 is removed by a cleaning blade 19 and is captured by bin 11. The image transferred to the paper 13 is fixed to the paper 13 by the fixation unit 12, and the paper 13 is then discharged out of the apparatus by the paper discharge unit 14.
The conventional image forming apparatus 10 constructed as described above is generally arranged in such a manner that the diameter ratio (I=D2/D1) between the diameter D1 of the development roller 5 and the diameter D2 of the subsidiary charge roller 8 is not more than 0.45. For example, if the diameter D1 of the development roller 5 is 16 mm, the diameter D2 of the subsidiary charge roller 8 is set to be 7 mm or less, whereby the diameter ratio I will be less than 0.45 (I=0.4375).
However, if the diameter ratio I between the development roller 5 and the subsidiary charge roller 8 is too small, in other words, if the difference between the diameters of the development roller 5 and the subsidiary charge roller 8 is too large, the amount of nip between the development roller 5 and the subsidiary charge roller 8 decreases as shown in FIG. 2. In this case, the developer 18 is unevenly charged, and the adhesive force is insufficient to attach the developer to the development roller 5 because the developer 18 does not have a sufficient length of time for charge, which causes the developer 18 to be scattered.
In addition, if the difference in diameter between the development roller 5 and the subsidiary charge roller 8 is too large, the angle will be abruptly changed from the ingoing area α to the outgoing area β of the nip between the development roller 5 and the subsidiary charge roller 8. Therefore, when developer 18 enterers the nip between the development roller 5 and the subsidiary charge roller 8, the developer 18 will not be evenly charged, whereas when the developer leaves the nip, there will not be sufficient time to remove the offset of developer from the subsidiary charge roller 8 which is produced in the nip. Therefore, the surface of the subsidiary charge roller 8 will become contaminated by the developer 18 when the apparatus is used over a long period, a result of which is the developer 18 will not be evenly charged.
Furthermore, if the diameter D2 of the subsidiary charge roller 8 is too small as compared to the diameter D1 of the development roller 5, the rotational speed of the subsidiary charge roller 8 becomes too rapid, thereby causing the developer 18 to be scattered.
Such unevenness of charge and scattering of the developer 18 will produce image defects such as background, reverse development or reverse transfer, white spots, black spots, and so forth, and can result in the contamination of the photosensitive conductor 1 and the subsidiary charge roller 8, thereby reducing the life-span of the development unit.
As shown in FIG. 3, in the conventional image forming apparatus 10, the subsidiary charge roller 8 comprises a rubber roller 8a and a metallic shaft 8b for fixing the rubber roller 8a. 
The rubber roller 8a is typically formed in such a manner that the diameter Dc of the longitudinal center part is larger than the diameter Ds of the opposite ends thereof. However, the diameter Dc of the longitudinal center part can be formed smaller than the diameter Ds of the opposite ends of the roller, depending upon fabrication requirements for the rubber roller.
Therefore, regardless of how the subsidiary charge roller 8 is formed, it is needed and serves to compress the subsidiary charge roller 8 against the development roller 5 with a predetermined line pressure in order to assure that the subsidiary charge roller 8 and the development roller 5 come into contact with each other while forming a predetermined nip. For that purpose, the subsidiary charge roller 8 is secured to a fixed frame (not shown) such that it comes into contact with the development roller 5 with a predetermined line pressure.
However, if such a line pressure is created having a value which is too small, when the subsidiary charge roller 8 comes into contact with the development roller 5 the nip between the subsidiary charge roller 8 and the development roller 5 can become uneven due to the constructional characteristics of the subsidiary charge roller 8 due to manufacturing tolerances. In this case, the diameters Dc and Ds of the longitudinal center part and opposite ends thereof can be formed slightly different, whereby a problem can arise in that developer 18 in a developer layer formed on the development roller 5 can then be unevenly charged.
Such an unevenness of charge of the developer layer can cause problems such as a decreasing image density when a solid image, such as a black dot, is printed under a low-temperature and low-humidity environment. Such an unevenness of charge can further reduce the adhesive force of developer 18 to the development roller 5 under a high-humidity environment, thereby causing the developer 18 to be scattered or deteriorating the reproducing capability for a fine image.
In addition, if a line contact pressure between the subsidiary charge roller 8 and the development roller 5 becomes too large due to manufacturing tolerance, the nip between the subsidiary charge roller 8 and the development roller 5 will become too small, whereby the developer 18 will undergo increased stress in the nip, although the evenness of charge is not affected by the line pressure. Consequently, the developer in an image formed thereby will have a reduced life-span and the image will deteriorate as time passes. Moreover, in this case, because the subsidiary charge roller 8 or the development roller 5 is excessively compressed, a filming phenomenon can result which renders some of components of the developer or rubber to be attached to any of the surfaces of the subsidiary charge roller 8 and the development roller 5, or to the relatively softer one of the two rollers 5 and 8, which will then be deformed thus changing its diameter. As a result, it becomes difficult to form a normal nip between the two rollers 5 and 8.
Furthermore, in the conventional image forming apparatus 10, the subsidiary charge roller 8 is constructed to be driven with a linear velocity equal with that of the development roller 5 by the contact frictional force between the subsidiary charge roller 8 and the development roller 5, which are in contact with each other with a predetermined line pressure.
Accordingly, developer 18 is subject to relatively intense pressure when it is positioned in the nip area between the subsidiary charge roller 8 and the development roller 5 because the linear velocities of the two rollers 5 and 8 are substantially equal from the initiating point to the ending point of the nip. Therefore, the developer 18 will undergo stress, whereby the developer in an image formed thereby will have a reduced life-span and the image will deteriorate as time passes, like the case in which a high line pressure is exerted as described above.
In addition, if the subsidiary charge roller 8 is driven by the contact frictional force with the development roller 5, the subsidiary charge roller 8 is prone to slip, without rotating along with the development roller 5 when the development roller 5 is rotated rapidly in a high speed image forming apparatus. In this case, developer 18 on the development roller 5 will not be evenly charged, thereby causing problems such as inferior images due to the reduced density of developer, the scattering of developer due to the increased speed of the development roller 5, and the reduced adhesive force of the developer 18. Further, if the development roller 5 rapidly rotates, the difference in angular displacement between the development roller 5 and the subsidiary charge roller 8 will be increased. Therefore, the compressive pressure acting on the subsidiary charge roller 8 and the development roller 5 will be increased, whereby the rollers 5 and 8 will be worn away when used over a long period, thus causing inferior rotation of the rollers 5 and 8.
Moreover, if scattered developer and/or foreign matter is introduced into bearings (not shown) which are provided for supporting the opposite ends of the shaft 8b of the subsidiary charge roller 8, the subsidiary charge roller 8 cannot be smoothly rotated or stopped via the contact frictional force with the development roller 5. If the subsidiary charge roller 8 is not smoothly rotated or stopped as described above, a part or all of the developer 18 cannot pass through the nip between the subsidiary charge roller 8 and the development roller 5. Therefore, problems can arise in that inferior developments such as decreased density, white band, and streak phenomena can be caused in the developing area between the development roller 5 and the photosensitive conductor 1. Additionally, still other problems can arise in that the developer may be accumulated between the developer layer regulation member 7 and the subsidiary charge roller 8, thereby causing a contamination of developer, or an increase of the driving load of the driving roller, in which case, the entire development unit 4 cannot be used.
Finally, if the bearings supporting the opposite ends of the shaft 8b of the subsidiary charge roller 8 become worn out due to long-term use, the distance between the axes of the subsidiary charge roller 8 and the development roller 5 will be changed, whereby a nip may become unevenly formed between the subsidiary charge roller 8 and the development roller 5.
Accordingly, a need exists for a system and method to maintain a desired nip and rotational speed between a subsidiary charge roller and a development roller, regardless of manufacturing tolerances and other factors, such that developer is evenly charged and properly carried.